Additive layer manufacturing (ALM) methods are known. In these methods a component is built up layer by layer onto a base plate until the 3D component is defined. In some ALM methods, layers are created by selective treatment of layers within a mass of particulate material, the treatment causing cohesion of selected regions of particulates into a solid mass. For example, the particulate is a ferrous or non-ferrous alloy powder and the treatment involves local heating using a laser or electron beam. Specific examples of such ALM methods include (without limitation); laser sintering, laser melting, direct laser deposition (DLD) and electron beam melting (EBM).
Additive layer manufacturing (ALM) techniques are known for use in defining complex geometries to high tolerances and can be used as an alternative to casting. For large components with more complex geometries it is known to provide an ALM pre-component which incorporates sacrificial support structures in addition to the designed geometry of the component. These structures hold overhanging parts of the pre-component in position during completion of the ALM step when powder layers above the support structure are being treated and treated layers are solidifying into the designed component geometry. Whilst such supports are helpful in preserving the designed geometry and preventing sagging of overhanging parts, such supports can be a source of cracking and crack propagation in the pre-component.
When the component is a high performance engineering component, for example, a component of a gas turbine engine, further treatment steps are necessary to address porosity and/or weaknesses in the component surface which might result in failure of the component under high pressure, high stress and/or high temperature conditions. Processes may include blasting or peening the surface. Hot Isostatic Pressing (HIP) is one commonly used process which may be used optionally in addition to a blasting or peening step. A component is subjected to both elevated temperature and isostatic pressure in a high pressure containment vessel. Typically this step is carried out in an inert gas environment to avoid any chemical reaction between the component and the pressurizing gas. A pre-component made by an ALM method is typically subjected to the HIP step with the base plate and support structures still in position. These assist in preserving the designed geometry of the component during the HIP process. After completion of the HIP step, sacrificial elements of the pre-component (such as the base plate and support structures) are removed. The surface of the remaining component may then be finished in one or more optional finishing steps.
A prior art method is now described with reference to the accompanying FIG. 2. In the described method, a component is manufactured from a high-temperature alloy by a Powder Bed Direct Laser Deposition (PB DLD) or Direct Metal Laser Sintering (DMLS) additive manufacturing process. A powder bed 1 is raised into the path of a spreading device 2 which spreads a thin layer of powder across a base-plate 3. The base-plate typically comprises a tool steel. Selected regions of the powder corresponding to a shape which it is intended to build are fused together (and also to the base-plate) by heat from laser 4. The base-plate 3 is gradually lowered with respect to the laser during the process enabling layer upon layer of powder to be applied and sintered by the laser. This layering process can create one or more components 5 simultaneously. Additional support material in the form of support element 6 is used to support the pre-component during the DLD build process, to improve conformity to the desired shape. The additional support material typically comprises the same material as the pre-component and is solidified as part of the ALM step to provide sacrificial support elements. The support elements aid in limiting sagging of overhanging shape elements and also serve to control the profile of the pre-component.
Subsequent processing steps are performed on the pre-component. Firstly, the pre-component is optionally blasted with a blast media (for example, beads) to create a compressed layer at the external surface. This compressed layer imposes a compressive force on underlying material resisting the propagation of cracks from within the main body of the pre-component. The pre-component and baseplate are then subjected to a Hot Isostatic Pressing step. The HIP process substantially eliminates micro-cracks in the material structure, so improving the properties of the pre-component material. This is a particularly important step for high-temperature nickel alloys (often used in the manufacture of gas turbine engine components), which when produced by DLD alone are very susceptible to cracking due to high micro-porosity and residual stresses in the pre-component. The HIP step compresses the pre-component whilst heating it. After the HIP process has been completed, structural elements and the base plate are removed, for example by a subsequent low impact machining step.